1. Field of the Invention
This invention relates to certain copolyesters which have been found to be useful as adhesives, especially fabric interlining adhesives.
2. Description of Prior Art
A fusible interlining is a material such as fabric which has been coated on one side with a discontinuous pattern of fusible adhesive. When the interlining is bonded to a face fabric in a garment, it provides body and shape to the garment without impairing the ability of the fabric to breathe. Fusible interlinings are used, for example, in the manufacture of men's and women's suits, in shirt collars and cuffs, and in the waistbands of trousers. In the manufacture of suits, polycarbonate basting threads are frequently used to temporarily hold the parts of the suit in place. After the suit is completed, a solvent such as perchloroethylene or trichloroethylene is used to embrittle the polycarbonate thread so that it may be brushed from the fabric.
Certain polyesters are useful for fusible interlining applications. However, these polymers also tend to have certain disadvantages. For example, one polyester of interest is the copolyester of terephthalic acid, adipic acid, ethylene glycol and 1,4-butanediol as described in U.S. Pat. No. 3,699,921. Such polyesters tend to block in pellet form and fumed silica must be added in significant amounts to make it possible to grind this polymer into powder. Excessive amounts of fumed silica in the powder, however, prevent good coatability and good fusion of the powders on the fusible interlining fabric when applied with powder point applicators.
The use of polyesters with a melting point of 160.degree. to 220.degree. C. which are composed of terephthalic acid plus, in some cases, isophthalic acid and one or more glycols having 2 to 10 carbon atoms is disclosed in U.S. Pat. No. 3,853,665. Ethylene glycol, propylene glycol, butylene glycol, pentanediols or hexanediols are proposed as the glycols. As glycol mixtures, those mixtures are used which contain ethylene glycol, examples being ethylene glycol with 1,6 hexanediol or ethylene glycol with 1,4 butanediol. Such copolyesters have the disadvantage in that they have relatively high melting points. They are therefore usable to only a limited extent as coating substances to be applied in the molten state or as fusion adhesives for heat-sensitive materials.
Copolyesters generally have lower melting points than homopolyesters. For example, the melting point of a polyester of terephthalic acid and ethylene glycol is around 260.degree. C. A polyester consisting of 90 mole % of terephthalic acid and 10 mole % of isophthalic acid in which ethylene glycol has been used as the diol component, has a melting point of 236.degree. C. When the molar ratio of terephthalic acid to isophthalic acid is 80:20, a copolyester is obtained which has a melting point of 210.degree. C. When the ratio of terephthalic acid to isophthalic acid is 70:30 the melting point drops to 185.degree. C.
Conditions are similar when ethylene glycol is replaced by 1,4 butanediol. A polybutylene terephthalate comparable to polyethylene terephthalate has a melting point of 225.degree. C.
In German "Offenelegunggaschrift" No. 1,920,432 there is disclosed a dry-cleaning fluid resistant polyester fusion adhesive prepared from (1) terephthalic acid and ethylene glycol, (2) adipic acid and 1,4-butanediol. The molar ratio of terephthalic acid to adipic acid ranges from a predominance of terephthalic acid to a predominance of adipic acid, and the molar ratio of ethylene glycol to 1,4-butanediol ranges from a predominance of ethylene glycol to a predominance of 1,4 butanediol. Such polyesters are used for bonding textiles. The copolyesters prepared in accordance with the patent has a softening point of 135.degree. C. This softening point, however, is still too high for many heat-sensitive materials which are to be laminated or are to be provided with a melted or sintered coating such as artificial leather, natural leather. If the molar ratios of the individual components of these copolyesters are varied, it is possible to arrive at a copolyester having a softening point of 110.degree. C. (terephthalic acid:adipic acid and molar ratio 60:40 with ethylene glycol:1,4 -butanediol ratio 60:40. The degree of crystallization of this copolyester, however, is already so low that it is not suitable for a fusion adhesive. Disadvantages reside in both the surface stickiness of the coated substrate and the stickiness of the copolyesters which is considerable even at room temperature. Copolyesters of this type are not suitable for the preparation of coating substances in powder form or adhesives in powder form such as required, for example, in the textile field.
It is well known in the art that the crystallinity of a polyester is one parameter which may be used to determine solvent resistance, i.e., the more amorphous (less crystalline), the more susceptible to dry-cleaning solvents the polyester will be. It is also known that the inherent property of glass transistion temperature is also a parameter by which the temperature at which a polyester, even an amorphous polyester, will be effected by a solvent.
It is also known that modification of a homopolyester by copolymerization with other acid or glycol moieties or combinations of glycol and acid moieties to form copolymers or terpolymers drastically reduces or eliminates crystallinity. The crystallinity of copolyesters is also dependent on the particular comonomers from which the copolyester is synthesized. For example, a polyester of terephthalic acid and 1,4 butanediol (even number of carbon atoms 4) will crystallize more readily than a polyester prepared from terephthalic acid and either 1,3 propanediol (odd number carbon atoms) or 1,5 pentanediol (odd number of carbon atoms). The crystallization phenomenon of copolyesters, especially those that are low melting, below 150.degree. C., is unpredictable.
Amorphous polyesters cannot be used as fusion adhesives in which resistance to dry-cleaning agents and high set-up speed are required. In like manner, those polyesters are undesirable which have too little crystallinity, because they solidify too slowly and consequently do not lose their surface stickiness for days or in many cases even weeks.
Since the requirements of crystallinity, set-up time, melting point and resistance to dry-cleaning are so numerous and are so dependent on so many interrelated and unrelated inherent monomer and polymer properties it is impossible to precisely design or produce useful copolyesters for these applications.
Other low melting adhesives that we are aware of are those disclosed in U.S. Pat. No. 4,094,721 and U.S. Pat. No. 3,948,859.